1. Field of the Invention
The invention relates to a method of measuring a load of a hoisting apparatus, where electricity is used as the driving force and a squirrel cage motor as the hoisting motor for moving a load attached to a hoisting member of the hoisting apparatus substantially in the vertical direction, the method comprising measuring the current and supply voltage of the hoisting motor and determining the stator winding resistance of the hoisting motor.
The invention further relates to an apparatus for measuring a load of a hoisting apparatus, where electricity is used as the driving force and a squirrel cage motor as the hoisting motor for moving a load attached to a hoisting member of the hoisting apparatus substantially in the vertical direction, the apparatus comprising means for measuring the current and supply voltage of the hoisting motor and a measuring member for measuring a variable describing the stator winding resistance of the hoisting motor.
2. Description of the Background Art
In hoisting apparatuses which are intended for vertical transfer of load and are typically either fixed or movable along a track by means of a trolley, determination of the load to be hoisted or lowered is very important for safety reasons, particularly for avoiding overloading of the hoisting apparatus. It is also necessary to know the cumulative amount of the load hoisted during the service life of the hoisting apparatus so as to anticipate the need for service of the hoisting apparatus or to determine safe service life. In prior art solutions the load of a hoisting apparatus can be measured either directly or indirectly. The hoisting apparatus load can be determined directly by arranging mechanical sensors, which measure the stretching or tension caused by the load, in the hoisting member of the hoisting apparatus or in another structure which supports the load to be hoisted or lowered. Use of mechanical sensors, however, increases the amount of work and costs both in the manufacture and modernization of the hoisting apparatus. Indirect measurement of the hoisting apparatus load is known from SE 454 625 and DE 19 617 105, where the hoisting apparatus load is determined on the basis of the input power of the hoisting motor. The solutions described in these publications also take the thermal losses generated in stator winding of the hoisting motor into account, i.e. it is either assumed that the losses are constant in the operating range of the hoisting motor or they are determined from the measured current and the stator winding resistance determined on the basis of the stator winding temperature. The stator winding resistance can be determined from the temperature of stator winding according to standard IEC34-1(-94), for example. The problem related to the solutions based on indirect load measurement is, however, that in changing operating conditions typical of hoisting apparatuses the hoisting apparatus load cannot be determined so accurately that reliable overload protection could be implemented for the hoisting apparatus.
The object of the present invention is to provide a new method and apparatus for determining the load of a hoisting apparatus.
The method of the invention is characterized by determining air gap torque of the hoisting motor, which describes the load of the hoisting apparatus, utilizing magnetization flux of the hoisting motor determined on the basis of the current, supply voltage and stator winding resistance of the hoisting motor and that the air gap torque is compared with an air gap torque curve determined for the hoisting apparatus at known reference loads to determine the load that corresponds to the air gap torque in question.
The apparatus of the invention is characterized in that the apparatus comprises a load measuring device for determining magnetization flux of the hoisting motor on the basis of the current, the supply voltage and a variable describing the stator winding resistance of the hoisting motor and for determining air gap torque of the hoisting motor, which describes the load, on the basis of the magnetization flux and that the load measuring device comprises means for comparing the air gap torque with an air gap torque curve determined for the hoisting apparatus at known reference loads to determine the load that corresponds to the air gap torque in question.
The basic idea of the invention is that in a hoisting apparatus where electricity is used as the driving force and a squirrel cage motor as the hoisting motor for moving a load attached to the hoisting member of the hoisting apparatus substantially in the vertical direction, the load of the hoisting apparatus is determined by first determining magnetization flux by means of the current, the supply voltage and a variable describing the stator winding resistance of the hoisting motor and then, on the basis of the magnetization flux, air gap torque which describes the hoisting apparatus load. After this, the air gap torque is compared with an air gap torque curve of the hoisting apparatus determined at known reference loads to determine the load corresponding to the air gap torque in question.
An advantage of the invention is that by using the magnetization flux of the hoisting motor for determining the air gap torque of the hoisting motor and thus for determining the load of the hoisting apparatus, the hoisting apparatus load can be determined with sufficient accuracy because the effects of varying operating conditions typical of hoisting operation can be clearly seen as changes in the magnetization flux of the hoisting motor. Thus the hoisting apparatus does not need to be provided with separate mechanical sensors to measure the load, and the accuracy of the load measurement result will be better than in prior art solutions employing indirect load measurement, which allows implementation of reliable overload protection for the hoisting apparatus.